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April 1999

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From:
"Stephen R. Gregory" <[log in to unmask]>
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Date:
Tue, 13 Apr 1999 11:19:44 EDT
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In a message dated 4/13/99 12:27:46 AM Pacific Daylight Time,
[log in to unmask] writes:

<< Can someone help with the processing of a heavily populated SM PCB,
 SM top and bottom which has to be reflowed both sides and has through
 holes parts ( no wave solder ).

 Any guidelines on how this should be processed would be of great
 help ie what side is the high temp solder used on, most densly least
 densly populated etc etc.

 Regards,
 ken Robertson
 Telspec >>

Hi Ken!

I'm assuming that the reason you must do double sided reflow is because the
board either has fine pitch or BGA's on both sides, am I correct? As far as
using different melting point solders, I know there's some info out there
that discusses that, but I really don't think that's a good thing to do. Just
say for instance; what if somebody screws up and prints the wrong paste on
the wrong side? You really don't need to do that anyway. Just about any
component I can think of (as long as IPC guidlelines have been followed as
far as pad layout and footprint) has enough surface tension to stay put when
solder goes liquidous on the bottom...(provided your board isn't bowing and
the reflow oven conveyer is smooth running).

The exceptions to that are extremely heavy parts like ceramic or metal bodied
QFP's and PLCC's, or parts with attached heatsinks. Sometimes Melfs will give
you problems, or I've had problems with 4-leaded plastic body oscillators and
certain filters too. Basically, suspect the parts that don't have much lead
area to 'em. The figure I remember, and many people say this is pretty
conservative, is that you'll have 30-grams of surface tension for every
square inch of lead surface.

As far as what side to do first, I usually leave the side that has the finest
pitch on it for the last side. If ya got any other specific questions, let us
know...

-Steve Greory-

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